fix(backend): preserve run message pagination boundary

.github/pull_request_template.md

@@ -58,4 +58,3 @@ Fixes #
        Backend:   cd backend  && make lint && make test
        Frontend:  cd frontend && pnpm format && pnpm lint && pnpm typecheck && BETTER_AUTH_SECRET=local-dev-secret pnpm build && make test
        Frontend E2E (if you touched frontend/): cd frontend && make test-e2e -->
-

backend/app/gateway/pagination.py

@@ -0,0 +1,20 @@
+"""Shared pagination helpers for gateway routers."""
+
+from __future__ import annotations
+
+
+def trim_run_message_page(rows: list[dict], *, limit: int, after_seq: int | None) -> tuple[list[dict], bool]:
+    """Trim a ``limit + 1`` run message page without dropping visible rows.
+
+    ``rows`` must be ordered oldest to newest. When an extra sentinel row is
+    present, latest-page and ``before_seq`` pagination receive the sentinel on
+    the older side, while ``after_seq`` pagination receives it on the newer side.
+    """
+    has_more = len(rows) > limit
+    if not has_more:
+        return rows, False
+
+    if after_seq is not None:
+        return rows[:limit], True
+
+    return rows[-limit:], True

backend/app/gateway/routers/runs.py

@@ -15,6 +15,7 @@
 
 from app.gateway.authz import require_permission
 from app.gateway.deps import get_checkpointer, get_feedback_repo, get_run_event_store, get_run_manager, get_run_store, get_stream_bridge
+from app.gateway.pagination import trim_run_message_page
 from app.gateway.routers.thread_runs import RunCreateRequest
 from app.gateway.services import sse_consumer, start_run, wait_for_run_completion
 from deerflow.runtime import serialize_channel_values
@@ -129,8 +130,7 @@ async def run_messages(
         before_seq=before_seq,
         after_seq=after_seq,
     )
-    has_more = len(rows) > limit
-    data = rows[:limit] if has_more else rows
+    data, has_more = trim_run_message_page(rows, limit=limit, after_seq=after_seq)
     return {"data": data, "has_more": has_more}
 
 

backend/app/gateway/routers/thread_runs.py

@@ -21,6 +21,7 @@
 
 from app.gateway.authz import require_permission
 from app.gateway.deps import get_checkpointer, get_current_user, get_feedback_repo, get_run_event_store, get_run_manager, get_run_store, get_stream_bridge
+from app.gateway.pagination import trim_run_message_page
 from app.gateway.services import sse_consumer, start_run, wait_for_run_completion
 from deerflow.runtime import RunRecord, RunStatus, serialize_channel_values
 
@@ -402,8 +403,7 @@ async def list_run_messages(
         before_seq=before_seq,
         after_seq=after_seq,
     )
-    has_more = len(rows) > limit
-    data = rows[:limit] if has_more else rows
+    data, has_more = trim_run_message_page(rows, limit=limit, after_seq=after_seq)
     return {"data": data, "has_more": has_more}
 
 

backend/tests/_run_message_pagination_helpers.py

@@ -0,0 +1,16 @@
+from fastapi.testclient import TestClient
+
+
+def assert_run_message_page(
+    client: TestClient,
+    url: str,
+    *,
+    expected_seq: list[int],
+    has_more: bool = True,
+) -> None:
+    response = client.get(url)
+
+    assert response.status_code == 200
+    body = response.json()
+    assert body["has_more"] is has_more
+    assert [m["seq"] for m in body["data"]] == expected_seq

backend/tests/test_runs_api_endpoints.py

@@ -5,6 +5,7 @@
 from unittest.mock import AsyncMock, MagicMock
 
 from _router_auth_helpers import make_authed_test_app
+from _run_message_pagination_helpers import assert_run_message_page
 from fastapi.testclient import TestClient
 
 from app.gateway.routers import runs
@@ -97,6 +98,51 @@ def test_run_messages_has_more_true_when_extra_row_returned():
     body = response.json()
     assert body["has_more"] is True
     assert len(body["data"]) == 50  # trimmed to limit
+    assert [m["seq"] for m in body["data"]] == list(range(2, 52))
+
+
+def test_run_messages_default_page_keeps_newest_messages_when_extra_row_returned():
+    """Default latest-page trimming drops the older sentinel row, not the newest message."""
+    rows = [_make_message(i) for i in range(16, 67)]
+    run_record = {"run_id": "run-2", "thread_id": "thread-2"}
+    app = _make_app(
+        run_store=_make_run_store(run_record),
+        event_store=_make_event_store(rows),
+    )
+    with TestClient(app) as client:
+        assert_run_message_page(client, "/api/runs/run-2/messages", expected_seq=list(range(17, 67)))
+
+
+def test_run_messages_before_seq_page_keeps_newest_side_when_extra_row_returned():
+    """Backward pagination trims the older sentinel so adjacent pages do not miss the boundary message."""
+    rows = [_make_message(i) for i in range(1, 18)]
+    run_record = {"run_id": "run-2", "thread_id": "thread-2"}
+    app = _make_app(
+        run_store=_make_run_store(run_record),
+        event_store=_make_event_store(rows),
+    )
+    with TestClient(app) as client:
+        assert_run_message_page(
+            client,
+            "/api/runs/run-2/messages?before_seq=18&limit=16",
+            expected_seq=list(range(2, 18)),
+        )
+
+
+def test_run_messages_after_seq_page_keeps_oldest_side_when_extra_row_returned():
+    """Forward pagination still trims the newer sentinel row."""
+    rows = [_make_message(i) for i in range(11, 62)]
+    run_record = {"run_id": "run-2", "thread_id": "thread-2"}
+    app = _make_app(
+        run_store=_make_run_store(run_record),
+        event_store=_make_event_store(rows),
+    )
+    with TestClient(app) as client:
+        assert_run_message_page(
+            client,
+            "/api/runs/run-2/messages?after_seq=10",
+            expected_seq=list(range(11, 61)),
+        )
 
 
 def test_run_messages_passes_after_seq_to_event_store():

backend/tests/test_thread_run_messages_pagination.py

@@ -6,6 +6,7 @@
 from unittest.mock import AsyncMock, MagicMock
 
 from _router_auth_helpers import make_authed_test_app
+from _run_message_pagination_helpers import assert_run_message_page
 from fastapi.testclient import TestClient
 
 from app.gateway.routers import thread_runs
@@ -88,6 +89,43 @@ def test_has_more_true_when_extra_row_returned():
     body = response.json()
     assert body["has_more"] is True
     assert len(body["data"]) == 50  # trimmed to limit
+    assert [m["seq"] for m in body["data"]] == list(range(2, 52))
+
+
+def test_default_page_keeps_newest_messages_when_extra_row_returned():
+    """Default latest-page trimming drops the older sentinel row, not the newest message."""
+    rows = [_make_message(i) for i in range(16, 67)]
+    app = _make_app(event_store=_make_event_store(rows))
+    with TestClient(app) as client:
+        assert_run_message_page(
+            client,
+            "/api/threads/thread-2/runs/run-2/messages",
+            expected_seq=list(range(17, 67)),
+        )
+
+
+def test_before_seq_page_keeps_newest_side_when_extra_row_returned():
+    """Backward pagination trims the older sentinel so adjacent pages do not miss the boundary message."""
+    rows = [_make_message(i) for i in range(1, 18)]
+    app = _make_app(event_store=_make_event_store(rows))
+    with TestClient(app) as client:
+        assert_run_message_page(
+            client,
+            "/api/threads/thread-2/runs/run-2/messages?before_seq=18&limit=16",
+            expected_seq=list(range(2, 18)),
+        )
+
+
+def test_after_seq_page_keeps_oldest_side_when_extra_row_returned():
+    """Forward pagination still trims the newer sentinel row."""
+    rows = [_make_message(i) for i in range(11, 62)]
+    app = _make_app(event_store=_make_event_store(rows))
+    with TestClient(app) as client:
+        assert_run_message_page(
+            client,
+            "/api/threads/thread-2/runs/run-2/messages?after_seq=10",
+            expected_seq=list(range(11, 61)),
+        )
 
 
 def test_after_seq_forwarded_to_event_store():